1. Field of the Invention
The present invention relates to an optical device, for example, a micro-Fresnel lens, a flat micro-lens, a grating device, an optical coupler, or an optical circuit and to a manufacturing method of the same.
2. Description of the Prior Art
FIGS. 1a to 1e are process diagrams schematically showing a conventional manufacturing process to fabricate an optical device (Fresnel lens).
On a surface of a silicon, Si substrate 11, an electron beam resist (e.g. CMS-EX(R): Negative-type regist) 12 is uniformely coated such that an electron beam, 13 is thereafter irradiated onto the resist 12 by use of an electron beam writing apparatus to draw a predetermined male stamper pattern FIG. 1a). Thereafter, the resist 12 is developed. As a result, a predetermined lens male stamper pattern 12a associated with the remaining resist layer remains on the substrate 11 (FIG. 1b). When the amount of irradiation of the electron beam is controlled depending on locations of the electron beam resist, the film thickness remaining after the development varies according to the locations in association of the amount of the irradiated electron beam. On the substrate 11 having the remaining resist 12a of the predetermined lens male stamper pattern thus formed, gold is evaporated to form an electrode layer 14 (FIG. 1c). Furthermore, chromium is plated thereon to form a plate layer 15 ( FIG. 1d). Thereafter, the remaining resist layer 12a and the silicon substrate 11, if necessary, are dissolving by use of an organic solvent, an etching agent, or the like to be removed so as to form a female stamper (an optical device stamper of a Fresnel lens or the like) 16.
A Fresnel lens is manufactured by use of the female stamper 16 thus prepared. Namely, by injecting an optical device organic material (ultraviolet (UV) setting or hardening resin or agent) 17 into a space between the male stamper 16 and a glass substrate 18 and then an ultraviolet ray is irradiated onto the organic material 17 by means of an ultraviolet (UV) lamp 19 to solidify the material 17, which is finally removed from the female stamper 16 to attain a plastic Fresnel lens (FIG. 1e). By use of the female stamper 16, a plurality of plastic lenses can be manufactured through the similar production process.
Incidentally, there have already been known a method of fabricating a stamper with a material transparent with respect to an ultraviolet ray (JP-A-1-180502) and a method in which a resin is injected onto a stamper without necessarily using an ultraviolet setting resin (JP-A-62-161532, JP-A-161533).
In accordance with the conventional optical device manufacturing method, since the optical device material is restricted by an organic material such as a plastic substance, there has been a disadvantage, for example, that the usage of the produced optical device is limited.
For example, since the organic materials for this purpose develop a refractive index of only about 1.5, the degree of freedom is narrowed for the design of the device. In addition, since the wavelength of light transmitting therethrough is in a range from about 0.4 micrometer to about two micrometers, the application field of the manufactured optical device is restricted. Furthermore, in the device production, air bubbles are likely to be produced and to enter the organic material, which leads to a problem that a satisfactory transcription cannot be developed when reproducing a fine pattern in a submicron range. Moreover, the refractive index greatly varies depending on temperatures, namely, "deviation" of the refractive index and a double refraction are likely to take place, which may possibly lower the performance of the optical device. In addition, when an organic material is used as the optical device material, the large variation in the volume occurs with a change of temperature, which leads to a deteriorated temperature characteristic. Furthermore, there exists a disadvantage that the material swells due to the characteristic of the organic material, namely, owing to high water absorption and a high hygroscopic property. Moreover, the heat resistivity is also unsatisfactory. As a result, in this situation, an ideal optical device cannot be produced.
In the conventional optical devices described above, for example, a flat micro-lens and a grating device, a lens layer or a grating layer is disposed on a glass substrate, namely, a fine surface having depressions and projections is exposed on the lens or grating layer. Consequently, the lens or grating layer surface is constantly brought into contact with the environmental air, and hence a satisfactory resistivity against dusts and humidity cannot be developed. Namely, dirts and dusts are fixed onto the fine surface of the lens layer, which leads to a disadvantage, for example, the lens characteristic is deteriorated.
In addition, although optical devices above have flat surfaces on the side of the glass substrate, the devices have fine shaped surface on the lens surface side. In consequence, a surface of another member can be brought into contact only with the flat surface side of the glass substrate, which lowers the degree of integration when the device is used. For example, in the conventional optical fiber coupler of FIG. 2, at top end portions of two optical fibers 20, connectors 22 are respectively attached such that these connectors 22 are inserted into a guide 21 so as to be fixedly secured thereon. Each connector 22 is provided with a Fresnel lens 23. These lenses 23 are linked to each other on the respective flat surfaces (JP-A-63-33717). The lenses 23 are disposed to oppose to the ends of the optical fiber 20 via a space. In this optical fiber coupler, the light propagates through the air and hence the coupler is unstable against a temperature change, namely, the coupler is not satisfactorily stable against changes in the environmental conditions. Moreover, there exists disadvantages, for example, since the Fresnel lenses are retained by a guide, the structure is also unstable.
On the other hand, as methods of coupling lights incident to a two-dimensional optical waveguide (layer) formed on a substrate, there have been known an objective method in which, as shown in FIG. 3, a light focused through an objective 27 enters an end surface of an optical waveguide 25 on a substrate 24 and an end coupling method in which, as shown in FIG. 4, an end surface of an optical fiber 28 is disposed in the proximity of an optical waveguide 25 or is fixedly attached onto the waveguide 25, or as shown in FIG. 5, a semiconductor laser 29 as a light source is disposed in the proximity of an optical waveguide 25 or is fixedly attached onto the waveguide 25.
However, in accordance with these optical coupling methods, the light entering an end surface of the optical waveguide 25 propagates and disperses through the waveguide 25. Consequently, the dispersed light is required to be collimated in the two-dimensional (2-D) optical waveguide 25. For this purpose, in the optical waveguide 25, a waveguide lens 26 is disposed as a collimator lens. However, since the waveguide lens 26 has not a satisfactory optical conversion or collimation efficiency, the incident light cannot be effectively utilized. Namely, the optical device including the optical waveguide 25 is attended with a considerable loss. Moreover, the size of the device is increased by the focal distance of the waveguide lens 26. In addition, in order to manufacture the waveguide lens 26 in the optical waveguide 25, it is necessary to employ manufacturing processes such as a sputtering, an etching, and a proton exchange and the manufacturing of the waveguide lens 26 is complicated. This reduces the yielding of the devices and hence soars the production cost. Furthermore, since the waveguide lens 26 is attended with a small allowance of positioning discrepancy or shift, the fabrication of the waveguide lens 26 is attended with difficulties.
As described above, since there exist many problems due to the waveguide lens, there has been required a development of an optical coupler unnecessitating the waveguide lens.
In addition, as the conventional composite optical devices, for example, a composite grating device, there has been known a device comprising a grating layer formed on the front and rear surfaces of a glass substrate and a device having a structure in which two glass substrates each having a surface on which a grating layer is formed are fixed to each other on a rear side thereof.
In the composite grating device of the prior art technology, the number of grating layers to form the composite device is limited to two, which leads to a disadvantage that the application fields are restricted. Furthermore, the composite grating device does not have a flat surface portion and hence cannot be employed to produce an integrated circuit together with other optical devices. Moreover, two glass substrates each having a grating layer cannot be easily fixed to each other by aligning the optical axis of each grating at a high precision.
Incidentally, there has been desired a Fresnel lens with a high numerical aperture (NA) as an objective for an optical pickup or a lens for a laser diode collimation. However, in a Fresnel lens, as the numerical aperture (NA) is increased, the minimum cycle or interval of the grating is reduced. The relationship between the numerical aperture and the minimum interval is represented as follows. EQU A=.lambda./NA
Where, A is a minimum interval and .lambda. indicates a wavelength.
An objective for a pickup requires a numerical aperture value not less than 0.45, and hence the minimum interval is to be about 1.7 micrometers. In consequence, the manufacturing precision is critical and there exist problems of the reduced efficiency due to the manufacturing error and of the occurrence of on-axis aberration (an aberration causes due to a discrepancy between the optical axis of the lens and the propagation direction of the incident light). Furthermore, as the interval is decreased, a discrepancy from a phase shift function (theoretical error) is increased, which leads to a deterioration of efficiency. As described above, it has been quite difficult to efficiently produce a high-NA Fresnel lens with a low aberration. Moreover, since the Fresnel lens is attended with a large off-axis aberration (which occurs in a location other than locations on the optical axis), the optical axis alignment is considerably difficult. For example, for a Fresnel lens having NA =0.45, in order to set the off-axis aberration to 0.01 or less, the angular discrepancy is required to be 0.016 degree or less (FIG. 35). FIG. 35 is a graph in which the numerical aperture (NA) and the angular discrepancy are respectively indicated along the abscissa and ordinate with a wave front aberration set as a parameter.
In consequence, heretofore, in an LD collimator having a beam shaping function primarily employed as a light source of a write-type optical disk, there is included, as shown in FIG. 6, a combination of a high-NA collimator lens 31 and a beam shaping prism 32. Lights from a semiconductor laser light source 33 disposed in the proximity of the focal point are collimated by the collimator lens 31 such that the collimated light beam is shaped into a circular contour by the beam shaping prism 32. However, in this constitution, the high-collimator lens and the beam shaping prism are expensive; moreover, the size of the collimator itself is disadvantageously increased.
Recently, in various optical devices, an optical circuit has been broadly adopted in which as shown in FIG. 7, an optical circuit device such as grating couplers 34 are disposed in an optical waveguide 25 on a substrate 24. For example, according to the constitution of this optical circuit, a light incident to a grating coupler 34 on one side is passed via an optical waveguide 25 so as to be emitted from a grating coupler 34 on the other side. In an optical device of this kind, after the optical waveguide is manufactured on the substrate, a complex production steps including the dry etching and the proton exchange are carried out to form the optical circuit devices. Consequently, the production of the optical circuit requires a considerable amount of work and time, which leads to problems that the mass production is difficult and that the production cost is soared.
Furthermore, because of the complicated manufacturing processes, the reproducibility of the devices is lowered and hence the performance of the products is decreased.
In addition, since the optical circuit elements or devices are fabricated after the optical waveguide 25 is manufactured, there has been problems that surfaces of the optical waveguide 25 are damages during the production and that the optical circuit devices cannot be easily aligned onto the optical waveguide 25.